1. Field of the Invention
The present invention relates to a propylene polymer composition. More specifically, it relates to a propylene polymer composition having resistance against radiation (i.e., high-energy radiation such as gamma rays and high-energy electron beam). The term "propylene polymer" used herein means polypropylene and propylene-ethylene copolymers.
2. Description of the Prior Art
Propylene polymer is widely used for formed or molded articles. When molded articles of propylene polymer are used as, for example, medical equipments or accessories, they may be exposed to radiation such as .gamma.-rays. Radiation exposure causes degradation of molded articles of propylene polymer. Thus, specifically, it remarkably decreases the tensile elongation of the articles, making the articles brittle, and causes yellowing and cracking.
Various attempts have been made to prevent the degradation of polyolefins due to radiation, as proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-19199, 58-42638, and 58-49737. These publications disclose the addition of hindered amine compounds or phenolic derivatives to resins. As mentioned above, when conventional propylene polymer compositions are subjected to, for example, 2.5 to 3.0 Mrad irradiation of cobalt-60, they are degradated and the elongation after the irradiation is remarkably decreased. However, when the above-proposed additives are incorporated into propylene polymer compositions, the desired radiation stability effect cannot be sufficiently exhibited. Furthermore, the commercial value of the molded articles obtained therefrom is often impaired due to the discoloration caused by the additives.